Generally an image forming apparatus such as a laser printer having its main body detachably provided with a belt unit has been put into practical use. For example, a main body of such image forming apparatus is provided with a support portion for supporting a support roller for the purpose of positioning. More specifically, the main body has a recess portion that accommodates the support roller for supporting the belt unit. A protrusion of the belt unit is inserted into the recess portion from one side so as to be attached as well as positioned. The aforementioned structure makes it possible to easily attach and detach the belt unit.
But, in the above-structured image forming apparatus, it is required to dispose the belt unit at the predetermined position in the main body at an appropriate tension with high accuracy for the purpose of stably performing the highly accurate paper feed and toner transfer, and thus it is not easy to use